Asp.net Membership Problem!

Problem:

Guid guid = new Guid(UserIDTextBox.Text.Trim()
);
MembershipUser user = Membership.GetUser(guid);
user.IsApproved = false;            
Membership.UpdateUser(user);
 i am getting exception at Membership.UpdateUser(user);
 Exception Details: System.Configuration.Provider.ProviderException: The user was not found.
 i find by debugging that user is a valid user


thanks ...